David Cameron: revelation came after the mayor of London, Boris Johnson, admitted he did not know the cost of a pint of milk. Photograph: REX/Ray Tang

David Cameron has been accused of being out of touch for not knowing how much it costs to buy a cheap loaf of bread – because he prefers to use an electronic breadmaker.

The prime minister said he thought a value loaf of supermarket white bread cost "well north of a pound", and then defended his ignorance of the correct price by saying he preferred to bake his own.

Cameron revealed he had a Panasonic breadmaker, which typically costs more than £100, and liked to use Cotswold Crunch flour, which is on sale online for £30.20 for a 16kg bag.

He stumbled over the question on the London radio station LBC 97.3, putting the Conservatives under further pressure over their response to squeezed household budgets.

"I don't buy the value sliced loaf, I've got a breadmaker at home which I delight in using and it turns out in all sorts of different ways," he said. "But you can buy a loaf in the supermarket for well north of a pound."

Told by presenter Nick Ferrari that the true cost was about 47p, he added: "Look, I'm trying to get my children to eat the sort of granary – and they take it actually, they like my homemade bread.

"A little plug for the flour made in my constituency – Cotswold Crunch – you get some of that, beautifully milled in the Cotswolds, you pop that in your breadmaker. You set the timer overnight so when you wake up there is this wonderful smell wafting through your kitchen.

"It takes 30 seconds to put in the ingredients. I'd recommend the Panasonic. There you are, that's a shameless plug. Very easy – even Nick Ferrari could work a Panasonic breadmaker."

The Tories are under pressure to do more to ease living costs after the Labour leader, Ed Miliband, pledged to freeze energy bills for 20 months if Labour returned to power.

Michael Dugher, a Labour shadow cabinet minister, mocked the prime minister for his response to the "cost of living crisis". "Next week on At Home With David Cameron: how much is a pint of milk, Dave? Dunno, I have my own dairy," he tweeted.

Mary Creagh, the shadow environment minister, said Cameron's advice sounded like "let them eat Cotswold Crunch" as children "show up hungry for school".

Another Labour MP, Kevin Brennan, said Cameron "gives a whole new meaning to upper crust".

The revelation came after the mayor of London, Boris Johnson, admitted he did not know the cost of a pint of milk. Johnson told Newsnight's Jeremy Paxman it was "about 80p or something like that". Told it was half that price, he conceded: "Well there you go, I don't know how much a pint of milk costs. So what?"

Their answers are likely to fuel the image of Cameron and his colleagues presented by Nadine Dorries, the Tory backbencher, who once described the prime minister and chancellor as "two posh boys who don't know the price of milk".

On Tuesday, Dorries was more on message, pointing out that no one tested the other political party leaders about the price of groceries.

"Knowing how the vast majority struggle and the cost of living should apply to every politician, not just Conservatives," she said. "Bread is cheap and healthy, one of my first lessons at school in home economics and it's delicious too – better without a bread maker."